In an application of a wireless communications technology, a stable reference frequency is very important. Generally, a crystal oscillator is used to generate a reference frequency of a reference clock and a phase-locked loop that are in a wireless radio frequency transceiver. In some communications protocols, such as in a GSM protocol, it is stipulated that, a difference between a frequency of a wireless portable device and a frequency of a base station needs to be within 0.1 ppm, which requires automatic frequency control (Automatic Frequency Control, hereinafter, AFC for short) on the crystal oscillator, where a signal for the AFC control is a digital signal. A digitally controlled crystal oscillator (Digitally Controlled Crystal Oscillator, hereinafter, DCXO for short) has the following characteristics: The DCXO is low in cost, and can be directly controlled by an external digital signal; and therefore the DCXO is widely used.
However, an oscillation amplitude value of the DCXO greatly affects performance of the DCXO. If oscillation amplitude is excessively large, the DCXO may stay in an overload state for a long time, causing reduced life time and even damage of the DCXO. If the oscillation amplitude is excessively small, noise performance of the DCXO may be affected. Generally, the oscillation amplitude of the DCXO is proportional to a bias current of the DCXO, and the oscillation amplitude of the DCXO may be controlled by changing the bias current of the DCXO. However, no technical solution for effectively controlling a bias current is provided in the prior art. In addition, for a voltage-controlled oscillator (voltage-controlled oscillator, hereinafter, VCO for short) frequently used in a phase-locked loop, oscillation amplitude of the VCO is also proportional to a bias current of the VCO. However, no technical solution for effectively controlling a bias current is provided in the prior art either.